The manufacturing of devices on semiconductor wafers consists of various repetitive steps, such as deposition of layers, etching of layers, polishing of layers, deposition and photolithography of photoresist layers and so on. After some of these steps, and when various parts or structures of the final device on the wafer are exposed, the semiconductor wafers need to be cleaned to remove particles which may have deposited during such various processing steps and before the next layer is deposited. As presently practiced, particle removal is usually achieved by a combination of mechanical and chemical mechanisms. In many wafer fabs, special dedicated tools are being used to scrub or remove particles, often called scrubbers. In many such scrubbers a mechanical particle removal method is used simply with room temperature or sometimes-heated DI (deionized) water. Chemicals can be added to enhance the removal efficiency. Cleaning tools differ from scrubbers in that mechanical removal techniques are combined with chemicals instead of simple DI water to remove particles both with a mechanical method and a chemical method combined. Additionally in a cleaning tool, other functions can also be performed in addition to particle removal. E.g. removal of metallic impurities, organic impurities and even wet etching of Si3N4, SiO2, poly-Si, Ni, Co, Ti, TiN or other layers and wet stripping of photoresist can be performed in a cleaning tool in addition to the particle removal function that is the object of this invention.
In a cleaning tool for semiconductor wafers, the most popular combination to remove particles is a combination of megasonics vibration together with a liquid mixture of NH4OH, H2O2 and H2O in which the wafers are immersed or with which the wafers are covered. The megasonics systems commonly used in the semiconductor industry use frequencies close to or about 1 MHz (one megahertz).
Before the invention of megasonics, ultrasonics was used. Ultrasonics has frequencies in the range of 20 kHz to about 120 kHz. However, since the invention of megasonics, ultrasonics is rarely used on semiconductor substrates because of the damage potential at frequencies in the range of 20 kHz to about 120 kHz. Hence, megasonics with frequencies close to or about 1 MHz is the current state of the art for semiconductor substrates.
The most popular chemistry conditions used for particle removal together with megasonics on semiconductor wafers, a liquid mixture of NH4OH, H2O2 and H2O, is a part of the so called RCA cleaning sequence originally developed by W. A. Kern and D. A. Puotinen, at the RCA company in 1965, and published in 1970, RCA Rev., 31, pp. 187-206 (1970). The traditional RCA cleaning sequence consists of 2 steps, the SC-1 step (Standard Clean-1 step) and the SC-2 step (Standard Clean-2 step). Specifically, the particle removal function of this RCA sequence is carried out by the SC-1 step of the RCA cleaning cycle. The SC-1 is also sometimes called the APM (Ammonia Peroxide Mixture) step. The SC-1 step (Standard Clean-1) is mainly aimed at removing particles while the SC-2 step (Standard Clean-2) is mainly aimed at removing metallic contamination. The SC-1 step consists originally of a 1/1/5 mixture of NH4OH (28-30% strength as NH3-w)/H2O2 (29-31% strength as H2O2-w)/DI (De Ionized) water at 70° C. There have been many variations on this recipe both in terms of mixing ratios and in terms of temperature.
Because of the importance of this particle removal step in the semiconductor industry, there has been a lot of research on the mechanism of this particle removal step. This chemistry can even be used without mechanical particle removal mechanism added. It is now widely accepted that, in case when there is no mechanical particle method added to the chemistry, and therefore, when particle removal is achieved by chemical contacting only, then the contacting chemistry simply removes particles due to underetching of the particle. A particular good publication on this mechanism was written by Hiroyuki Kawahara, K. Yoneda, I. Murozono and T. Tokokoro, “Removal of Particles on Si Wafers in SC-1 solution”, IEICE Trans. Electron., Vol. E77-C, No. 3, March 1994, p. 492. The underetching theory goes as follows: a controlled amount of the surface layer is uniformly removed or etched all over the surface of the wafer to be cleaned. When etching this surface layer, the material underneath the particle is also etched away and this etching releases the particle from the surface. Then, the particle is washed away.
Since the current state of the art for removing particles by chemical means only, relies on undercut etching, and since etching increases with temperature, everyone so far has found that particle removal efficiency increases with temperature. M. Meuris et al., Microcontamination, May 1992, p. 31, e.g. showed the effect of temperature on etching rate. Therefore, SC-1 solutions are usually used at elevated temperatures. Increased etching can also be achieved by using higher chemical concentration. If the etching is too excessive for the device or substrate at hand, then sometimes the temperature is lowered, but the particle removal efficiency is then reduced as well.
In the prior art, when SC-1 solutions are used without any mechanical particle removal method, i.e. by chemical contacting alone, the temperatures used always range from room temperature up to about 80° C. If there is no reliance on the chemical undercutting and when relying on a mechanical method to remove particles instead, then the high temperature is not needed. For example, when using SC-1 together with megasonics to remove particles, the SC-1 or any alternative high pH chemistry, merely serves to prevent particles from re-depositing. When using megasonics, the particles are removed by the megasonics vibration through the formation of cavitation events and are prevented from reattaching to the surface by the high pH of the SC-1 chemistry or alternative high pH chemistry.
Since megasonics relies on cavitation to remove particles and since cavitation is not very much temperature dependent, but very dependent on the dissolved gases, it has been found that megasonics and SC-1 combined don't remove any particles when there are no gases present. It is possible to create cavitation without any dissolved gases present, so called vacuum cavitation, but this is only possible at very high power levels. Hence, at normal power levels, typically 10-100 W, there is no gaseous cavitation when there are no dissolved gases present and hence there is virtually no particle removal efficiency when using megasonics without any gases dissolved. In R. Gouk, J. Blocking, S. Verhaverbeke, “Effects of Cavitation and Dissolved Gas Content on Particle Removal in Single Wafer Wet Processing”, in Proceedings of Semiconductor Pure Water and Chemicals Conference (SPWCC) 2004, Santa Clara, Calif., 2004, it is shown that at 925 kHz and for power levels up to 0.15 W/cm2 (corresponding to 100 W on a 300 mm wafer), there is absolutely no particle removal efficiency for dissolved gas levels of 30 ppb O2. Only for power levels starting at 0.3 W/cm2 (200 W), the particle removal efficiency starts to become non-zero (20%). However, even at only 100 W and with 300 ppb of dissolved gas (O2 in this case), the particle removal efficiency is 90%. This shows clearly the effect of dissolved gas on the particle removal efficiency due to the cavitation of dissolved gas bubbles.
Currently, a cleaning paradox has emerged. Megasonics vibration works well for removing particles and with a very wide temperature range, but the cavitation which the megasonics produces, and which is used to remove particles, also damages the fine patterns on the wafers. Indeed, the patterns on the wafer are becoming so small that they are very fragile and are very prone to mechanical damage. This started to be a problem when the pattern size on the wafers decreased to a size smaller than 0.3 μm in width or at least in 1 dimension. Initially it was addressed by lowering the megasonics power, but now with pattern sizes sometimes as small as 22 nm in 1 dimension, any megasonics power or rather any cavitation will destroy such patterns.
Therefore, a new method for removing small particles from the front side of the wafer without damaging the fragile structures is necessary. The underetching mechanism, which does not damage the fragile structures, however can also not be used anymore, since the devices are so small, that underetching would remove valuable material from the surface of the device and hence the device characteristics would be degraded. This is the current cleaning paradox that we are faced with.
This is the case, because the current generations of small devices have the active device region extremely close to the top surface. This is very clearly shown for the case of semiconductor wafers and devices made on such wafers, in the publication by F. Arnaud, H. Bernard, A. Beverina, E. El-Frahane, B. Duriez, K. Barla and D. Levy, “Advanced Surface Cleaning Strategy for 65 nm CMOS device Performance Enhancement”, Solid State Phenomena Vols. 103-104 (April 2005) pp. 37-40. In this publication, F. Arnaud et al. clearly show that reducing the underetching enhances the transistor characteristics.
Hence the paradox: mechanical particle removal cannot be used anymore for small particle removal, since it also damages the fine patterns, and conventional chemical particle removal by underetching cannot be used anymore, because of loss of surface material which is now a substantial part of the device.
Even in those cases where the substrate is completely flat and where damage is not a paramount concern, it has been found that for very small particles, the mechanical methods are not effective anymore. This is shown in G. Vereecke, F. Holsteyns, S. Arnauts, S. Beckx, P. Jaenen, K. Kenis, M. Lismont, M. Lux, R. Vos, J. Snow and P. W. Mertens, “Evaluation of Megasonic Cleaning for sub-90 nm Technologies”, Solid State Phenomena Vols. 103-104 (2005) pp. 141-146. Mechanical methods to remove particles, such as, but not limited to, brush scrubbing, spray aerosol bombardment, and ultrasonic and megasonic vibration, are very effective for the large particles, i.e. for particles >90 nm, but loose efficiency for the very small particles, <90 nm. Hence, there is a need for an improved chemical method to remove these very small particles even on substrates without exposed patterns. This is true for wafers with devices on them after CMP (Chemical Mechanical Polishing). Indeed, these wafers contain parts of the device already on them, but have been flattened or polished without removing all of the layers in order to make it easier for photolithography to pattern the next layer. On such wafers, very small particles are difficult to be removed with conventional mechanical techniques. Gross etching on such polished wafers is not possible, since then the layers which are exposed would be etched away. These layers will become part of the device later and cannot be etched substantially.
As a summary, there is a great need in the semiconductor industry for a solution and a method and an apparatus that can remove small particles from the front side or back side of semiconductor wafers without damaging the fine patterns and without substantial underetching of the surface material. There is a general need for an improved chemical method to remove very small particles (<90 nm) even on substrates without pattern such as wafers with partial devices after chemical mechanical polishing.
More generally, none of the presently known methods can remove efficiently very small particles, since most of the mechanical techniques loose efficiency for small particles and most of the currently known chemical methods are not very effective for very small particles. The prior art does not provide for an improved chemical method to remove very small particles more efficiently than the ubiquitous SC1 solution for wafers with partial structures or layers on them.